Conventionally, a medium-wave broadcast transmitter uses a digital amplitude modulation device.
The digital amplitude modulation device includes multiple power amplifiers that are connected in parallel.
By switching control (such as controlling on/off) of the power amplifiers according to a voltage amplitude level of a modulation signal such as a sound signal provided externally, a carrier signal is amplified while the number of power amplifiers to be concurrently placed in ON state is changed.
Output signals from the power amplifiers in on the concurrent ON state are then combined to generate an amplitude modulation (AM) wave, and the generated AM wave is transmitted to a predetermined broadcast service area.
In this type of digital amplitude modulation device, the signal quality of AM waves may be deteriorated when some kind of failure, such as breakage of a power amplifier, or short circuit or disconnection of a joint portion, occurs.
Therefore, it is necessary to prevent the power amplifiers from being damaged due to, for example, a lightning strike, a load-related anomaly such as disconnection, corruption, and entry of a surge, and degradation in a standing wave ratio (SWR) by a load characteristic of an antenna.
For calculation of an SWR, a micro processing unit (MPU) is generally used. However, an MPU has a large calculation delay amount and cannot follow a sudden change in the SWR in actual time at the time of a lightning strike or the like.
This makes it difficult to prevent the power amplifiers in the digital amplitude modulation device from being damaged. Moreover, the same problem arises in a digital circuit including a complex programmable logic device (CPLD) or a field programmable gate array (FPGA) for the control to prevent a damage of power amplifiers.
Accordingly, an object of the present embodiment is to provide a digital amplitude modulation device and a digital amplitude modulation control method that can follow a sudden change along with occurrence of an anomaly in actual time.